The invention relates to modems.
A modem (xe2x80x9cmodulator-demodulatorxe2x80x9d) is the electronic device that sends digital information, typically from a computer, over an analog communication channel, for instance, a telephone line. In a typical transmit application, a modem accepts digital data from a computer, and generates analog waveforms that can be carried by a telephone line. In a receive role, the modem accepts analog waveforms from the line and produces digital data for use by the computer. The protocol for translating digital data to and from analog waveforms is agreed among modem manufacturers.
Modem transmit protocols require continuous control of the analog output: if the transmit circuitry does not keep up with the continuous process of translating digital data to analog waveforms, the resultant departure from the protocol can result in a retraining of the modems, resulting in transmission delays or the line being hung up.
As the speed of computer central processing units (CPU""s) has increased, the signal processing functions of modems have been moved from dedicated hardware to the CPU. If that CPU is delayed in servicing a request from the transmit circuitry of the modem, the consequences of the previous paragraph result.
In general, in a first aspect, the invention features a technique for use in a computer modem. A modem transmit data channel is monitored for a data starvation condition. When the data starvation condition is detected, a data sequence is synthesized on a receive channel of the modem, the synthesized data sequence being essentially independent of data received on a receive terminal of the modem.
In general, in a second aspect, the invention features a technique for use in a computer modem. Parameters of an echo cancellation filter are adapted, responsive to data transmitted over a modem transmit channel and received on a modem receive channel. When the transmit channel is disrupted, a data sequence is synthesized in the receive channel, the data of the synthesized sequence being based on the adapted parameters of the echo cancellation filter.
Preferred embodiments of the invention may include one or more of the following features. The synthesized receive data sequence may be synthesized to approximate an echo of data transmitted on the transmit data channel. The synthesized approximate echo may reflect an intentional misadjustment. The misadjustment may be induced by ignoring components of the data sequence received from a far modem, for instance the message data from the far modem, or a far modem echo. The intentional misadjustment may be induced by configuring a filter with a number of taps insufficient to model an impulse response of a round-trip over a transmission channel from the transmit data channel to the receive channel. The intentional misadjustment may be induced by selecting a filter adaptation parameter to preserve a misadaptation of the filter, for instance, the filter""s adaptation mu parameter. The data starvation condition may be detected when a CODEC demands data from an empty transmit data buffer.
In general, in a third aspect, the invention features a technique for use in a computer modem. When a data starvation condition is detected on a modem transmit data channel, a waveform sample sequence is synthesized for transmission on a transmission channel of the modem, the synthesized waveform sample sequence being essentially free of timing information.
Preferred embodiments of the invention may include one or more of the following features. The synthesized data sequence may be synthesized by averaging the values of several samples from a generated transmit sample sequence. The number of samples averaged may correspond to an oversampling factor of the modem transmit data channel. The samples averaged may be drawn to provide roughly equal representation of all phases of the transmit signal.
Particular embodiments of the invention may feature one or more of the following advantages. The numerically-intensive signal processing and logically-intensive protocol management of a modem process can both be performed in the microprocessor of a computer system, reducing system cost. Delays in real time response of the microprocessor are less disruptive of modem communications. Retraining of the receive channel of a local or remote modem can be reduced. Hang ups of the modem can be reduced. A synthesized transmission can force a receive error, which will in turn force a retransmit under controlled conditions. A synthesized signal that allows the remote receiver""s timing recovery process to coast is less likely to cause the remote receiver to diverge its timing synchronization from the transmitter during periods of signal substitution.
The above advantages and features are of representative embodiments only, and are presented to assist in understanding the invention. It should be understood that they are not to be considered limitations on the invention as defined by the claims, or equivalents to the claims. No single advantage should be considered necessary for equivalence. Additional features and advantages of the invention will become apparent in the following description, from the drawings, and from the claims.